This invention is concerned with techniques for thermal management of electronic devices and more particularly with high heat flux cooling technology for microelectronic systems.
Both the performance reliability and life expectancy of electronic equipment are inversely related to the component temperature of the equipment, with a reduction in the temperature corresponding to an exponential increase in the reliability and life expectancy of the device. Therefore, long life and reliable performance of a component may be achieved by effectively controlling the device operating temperature within the design limits for the device. One of the primary devices employed for heat dissipation in microelectronic systems is a heat sink, which absorbs and dissipates heat from a microelectronic device using thermal contact, either direct or radiant. The heat sink is typically a metal structure in contact with the electronic component's hot surface, though in most cases a thin thermal interface material mediates between the two surfaces. Microprocessors and power handling semiconductors are examples of electronics that need a heat sink to reduce their temperature through increased thermal mass and heat dissipation, primarily by conduction and convection and, to a lesser extent, by radiation.
Heat sinks function by efficiently transferring thermal energy from an object at a relatively high temperature to a second object that is at a relatively lower temperature and that has a much greater heat capacity. The goal is to effect a rapid transfer of thermal energy that quickly brings the high temperature object into thermal equilibrium with the low temperature object. Efficient functioning of a heat sink relies on the transfer of thermal energy from the first object to the heat sink at a high rate and from the heat sink to the second object. The high thermal conductivity of the heat sink material, combined with its large surface area (often provided by an array of comb or fin like protrusions), results in the rapid transfer of thermal energy to the surrounding cooler air. Fluids (such as refrigerated coolants) and thermally efficient interface materials can ensure good transfer of thermal energy to the heat sink. Similarly, a fan may improve the transfer of thermal energy from the heat sink to the air.
Heat sink performance, by mechanisms including free convection, forced convection, and liquid cooling, is a function of material, geometry, and the overall surface heat transfer coefficient. Generally, forced convection heat sink thermal performance is improved by increasing the thermal conductivity of the heat sink materials, increasing the surface area (usually by adding extended surfaces, such as fins or foamed metal) and by increasing the overall area heat transfer coefficient (usually by increasing the fluid velocity, by adding fans, coolant pumps, etc.). In addition, heat sinks may be constructed of multiple components exhibiting desirable characteristics, such as phase change materials, which can store a great deal of energy due to their heat of fusion.
When the microelectronic device is substantially smaller than the base plate of a heat sink, there is an additional thermal resistance, called the spreading resistance, which needs to be considered. Performance figures generally assume that the heat to be removed is evenly distributed over the entire base area of the heat sink and thus do not account for the additional temperature rise caused by a smaller heat source. This spreading resistance could typically be 5 to 30% of the total heat sink resistance.
Heat pipes are another useful tool that in the thermal management of microelectronics. A heat pipe can transport large quantities of heat between hot and cold regions with a very small difference in temperature. A typical heat pipe consists of a sealed hollow tube made of a thermoconductive metal such as copper or aluminum. The pipe contains a relatively small quantity of a working fluid, such as water, ethanol or mercury, with the remainder of the pipe being filled with the vapor phase of the working fluid. The advantage of heat pipes is their great efficiency in transferring heat.
The demands made on the thermal management of microelectronic systems are increasing with smaller form factors, elevated power requirements and increased bandwidth being established for next generation electronic systems. High power density, wide bandgap technology, for example, exhibits an extremely high heat flux at the device level. In addition, composite structures have low thermal mass and are not effective conductors of heat to heat sinks. The design of low cost COTS (commercial off the shelf) electronics frequently increases heat dissipation, and modern electronics is often packaged with multiple heat sources located close together. Some systems have local hot spots in particular areas, which induce thermal stress and create performance degrading issues.
These changes are resulting in an increase in the average power density, as well as higher localized power density (hot spots). As a result, the dissipation power density (waste heat flux) of electronic devices has reached several kwatts/cm2 at the chip level and is projected to grow much higher in future devices. Management of such power densities is beyond the capability of traditional cooling techniques, such as a fan blowing air through a heat sink. Indeed, these power densities even exceed the performance limits of more advanced heat removal techniques, such as a liquid coolant flowing through a cold plate. A common practice to address heat spreading issues is to adopt highly conductive bulk materials or to incorporate a heat pipe as the heat spreader. These approaches, however, involve heavy components, the thermal conductivity may be too low, mechanical strength can be a limiting factor, and the heat flux may be too low. Consequently, some new electronic devices are reaching the point of being thermally limited. As a result, without higher performance thermal management systems, such devices may be hampered by being forced to operate at part of their duty cycle or at a lower power level.
Improvements are needed to increase the heat transfer coefficient, as well as to reduce the spreading resistance, primarily in the base of the heat sink. Advanced high heat flux liquid cooling technologies, based on phase change heat transfer, are needed to satisfy requirements for compact, light weight, low cost, and reliable thermal management systems.